Gas exchange system flow configuration

ABSTRACT

Active compensation designs to offset the impact of gas diffusion sources and sinks in a photosynthesis and transpiration measurement system are disclosed. A sensor head for use in a gas exchange analysis system includes an active, piezoelectric flow splitting device for splitting a flow between a sample chamber and bypass pathway. The active flow splitting device is controlled by feedback from a downstream flow meter. A continuous measurement system for rapidly and accurately surveying large numbers of samples is described.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims the benefit of U.S. Non-Provisional applicationSer. No. 13/240,613, [Attorney Docket No. 510341 (012910US)], andProvisional Application No. 61/385,909, [Attorney Docket No.85409-792381(012900US) (formerly 020031-012900US)], filed on Sep. 23,2010, the disclosure of which is incorporated herein by reference in itsentirety.

This application is related to U.S. application Ser. No. 13/149,709,[Attorney Docket No. 85409-790436(010210US)], filed May 31, 2011, whichis a continuation-in-part of U.S. application Ser. No. 12/889,289,[Attorney Docket No. 85409-781419(010200US) (formerly 020031-010200US)],filed on Sep. 23, 2010, the disclosures of which are incorporated hereinby reference in their entireties.

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSOREDRESEARCH OR DEVELOPMENT

NOT APPLICABLE

BACKGROUND

The present invention relates generally to gas exchange measurementsystems, and more particularly to open photosynthesis measurementsystems having an optimized flow configuration to minimize errorsresulting from gas diffusion.

Systems for measuring plant photosynthesis and transpiration rates canbe categorized as open or closed systems. For open systems, the leaf orplant is enclosed in a chamber, and an air stream is passed continuouslythrough the chamber. CO₂ and H₂O concentrations of chamber influent andeffluent are measured, and the difference between influent and effluentconcentration is calculated. This difference is used, along with themass flow rate, to calculate photosynthesis (CO₂) and transpiration(H₂O) rates. For closed systems, the leaf or plant is enclosed in achamber that is not supplied with fresh air. The concentrations of CO,and H₂O are continuously monitored within the chamber. The rate ofchange of this concentration, along with the chamber volume, is used tocalculate photosynthesis (CO₂) and transpiration (H₂O) rates.

In both open and closed systems, it is important that the leaf or plantbe the only source or sink of both CO₂ and H₂O. CO₂ or H₂O concentrationchanges not caused by the plant are a measurement error. These errorscan be empirically compensated, for example as described in the LI-CORBiosciences LI-6400 User Manual (pp. 4-43 thru 4-48). Some instrumentusers may not understand the significance of these corrections, andneglect them.

Both open and closed systems contain a circuit of pneumatic components(e.g., pumps, valves, chambers, tubing, analyzers, etc.). When CO₂ andH₂O concentrations are dynamically changing, sorption on thesecomponents can provide an apparent CO₂ or H₂O source and/or sink. Understeady-state conditions, sorption is not an active source or sink, andparasitic CO₂ and H₂O sources and/or sinks can be attributed to bulkleaks and diffusion.

Bulk leaks are driven by pressure differentials between the system andthe ambient environment. Proper system design and construction, alongwith inherently low operating pressures, generally minimize parasiticsources and sinks due to bulk leaks. Diffusion is driven by constituentgas (CO₂ and H₂O) concentration gradients between the system and ambientenvironment. Any time constituent gas concentrations inside the systemare significantly different than ambient conditions, the diffusionpotential increases. Metals, in nearly any practical working thickness,generally provide an outstanding diffusion barrier to gases.Practically, however, nonmetallic materials are always required. Forexample, to provide a seal between metallic materials, gaskets andO-rings are used Flexible tubing which connects the sensor head to othersystem components is an example of functional capabilities which cannotbe reasonably achieved with metals.

In open photosynthesis systems, a conditioned air stream is typicallysplit into two streams. FIG. 1 a illustrates the flow path in such anopen system where the flow is split at the console, remote from thesensor head, and flows to the sensor head via two separate paths. Thefirst flow path (known as reference) passes through a gas analyzer(e.g., Infra-Red Gas Analyzer or IRGA) which measures constituent gasconcentrations (CO₂ and H₂O). The second flow path (known as sample)passes through a sample chamber (leaf chamber) in which gas exchangeoccurs. This second sample flow path exits the chamber and enters asecond gas analyzer (e.g., IRGA). The difference between the sample andreference gas concentrations is used to calculate photosynthesis (CO₂)and transpiration (H₂O). As photosynthesis and transpirationmeasurements are based on concentration differences in these two gasstreams, the accuracy in measuring the difference is more critical thanmeasuring the absolute concentration of either. Diffusive parasiticsources and/or sinks present in the tubing, connectors, and fittingsthat supply the head with the sample and reference gas streams cancompromise measurement accuracy.

In practice, it is nearly impossible to fully eliminate parasiticsources and sinks due to diffusion. Therefore it is desirable to providesystems and methods that minimize the impact of diffusion and that helpovercome the above and other problems.

BRIEF SUMMARY

The present invention provides systems and methods for measuringphotosynthesis and transpiration rates.

Embodiments of the invention provide system flow path designs that helpminimize the impact of diffusion. By reducing the magnitude of parasiticsource and sinks, lower rates of photosynthesis and transpiration can bemore accurately measured, e.g., without the need for extensive empiricalcompensation. Other embodiments provide systems and methods forperforming survey measurements of gas concentration and also gasanalyzers having piezoresistive or other active flow splitting devicesthat variably split a received gas flow to two gas outlet ports orchannels.

According to one embodiment, a sensor head for use in a gas exchangemeasurement system is provided. The sensor head typically includes asample chamber defining a measurement volume for analysis of a sample,the sample chamber having an inlet and an outlet, and a flow splittingmechanism located proximal to the sample chamber, the mechanismconfigured to split a gas flow received at an input port from a remotesource to a first output port and to a second output port, wherein thefirst output port is coupled with the inlet of the sample chamber. Thesensor head also typically includes a first gas analyzer coupled withthe outlet of the sample chamber and configured to measure aconcentration of one or more gases, and a second gas analyzer coupledwith the second output port of the flow splitting mechanism andconfigured to measure a concentration of the one or more gases.Advantageously, gas diffusion sources and sinks, which differentiallyaffect gas concentrations, are reduced due to the proximity of the flowsplitting mechanism with the sample chamber and gas analyzers. Thisadvantageously reduces measurement error associated with or attributableto gas diffusion sources and sinks. The proximity advantage derives fromminimizing the joints, gaskets, fittings, tubing lengths, and materialsall prone or susceptible to gas diffusion. In certain aspects, the oneor more gases measured by the first and second gas analyzers includesCO2 or H2O.

According to another embodiment, a method is provided for measuring agas concentration differential in a gas exchange analysis system havinga sensor head having a sample chamber defining a measurement volume forsample analysis, the sample chamber having an inlet and an outlet, and aflow splitting mechanism located proximal to the sample chamber. Themethod includes splitting a gas flow received from a remote source at aninput port of the flow splitting mechanism to a first output port and toa second output port, wherein the first output port is coupled with theinlet of the sample chamber, measuring a concentration of one or moregases exiting the sample chamber using a first gas analyzer, andmeasuring a concentration of the one or more gases exiting the secondoutput port of the flow splitting mechanism using a second gas analyzer.The method also includes determining a concentration differential of theone or more gases based on the first concentration and the secondconcentration, whereby measurement errors associated with diffusionsources and sinks of said gas are reduced due to the proximity of theflow splitting mechanism to the sample chamber and gas analyzers. Incertain aspects, the measured gases include CO₂ or H₂O.

According to yet another embodiment, a device is provided for variablysplitting the flow of gas in a sensor head of a gas exchange analysissystem. The device typically includes an input port, a first outputport, a second output port and a flow splitting mechanism, the devicebeing located proximal to a sample analysis chamber having a measurementvolume, and gas analyzers. The flow splitting mechanism is typicallyconfigured to variably split a gas flow received at the input port froma remote source to the first output port and to the second output port,wherein the first output port is coupled via a flow path to an inlet ofthe sample analysis chamber. The flow volume created by the measurementvolume and the flow path is sufficiently small such as to reduce thetime required to reach a steady state of gas concentrations in the flowvolume when a flow ratio to the flow path is adjusted in the flowsplitting mechanism. In certain aspects, the flow splitting mechanism isconfigured to variably adjust the flow ratio such that the gas flow isabout 0% to 100% to the first output port and the remaining 100% to 0%to the second output port.

According to a further embodiment, a sensor head for use in a gasexchange analysis system is provided that typically includes a samplechamber defining a measurement volume for analysis of a sample, thesample chamber having an inlet and an outlet, and a piezoresistive flowsplitting device located proximal to the sample chamber, the deviceconfigured to variably split a gas flow received at an input port to afirst output port and to a second output port, wherein the first outputport is coupled with the inlet of the sample chamber. The sensor headalso typically includes a first gas analyzer coupled with the outlet ofthe sample chamber and configured to measure a concentration of a gas, asecond gas analyzer coupled with the second output port and configuredto measure a concentration of said gas, and a flow meter coupled betweenthe first output port and the inlet of the sample chamber or between thesecond output port and the second gas analyzer, the flow meter beingadapted to measure a flow rate. The sensor head further typicallyincludes a feedback control circuit adapted to control thepiezoresistive flow splitting device to adjust a ratio of gas flow tothe first and second output ports responsive to a flow rate signal fromthe flow meter. En certain aspects, the piezoresistive flow splittingdevice is configured to variably adjust the flow ratio such that the gasflow is between about 0% to 100% to the first output port andconcomitantly between about 100% to 0% to the second output port.

According to yet a further embodiment, a method is provided formeasuring a concentration of a gas in a gas exchange analysis systemhaving a sample chamber defining a measurement volume for analysis of asample, the sample chamber having an inlet port coupled with a gassource and an outlet port. The method typically includes measuring afirst concentration of a gas at the input port at each of a plurality oftimes, measuring a second concentration of said gas at the output portat each of said plurality of times, and thereafter determining at eachof said plurality of times a concentration differential between thefirst measured concentration and the second measured concentration andintegrating the concentration differential over time.

According to yet another embodiment, an open-path gas exchange analysissystem is provided that includes an enclosed sample chamber defining ameasurement volume for analysis of a sample, with the sample chamberhaving a gas inlet port coupled with a gas source and a gas outlet port.The system also includes a first gas analyzer configured to measure afirst concentration of a gas entering the gas inlet port, a second gasanalyzer configured to measure a second concentration of said gasexiting the gas outlet port at the plurality of times, and a processingmodule configured to determine at each of the plurality of times aconcentration differential between the first measured concentration andthe second measured concentration, and to integrate the concentrationdifferential over time. In certain aspects, the measured gas includesCO₂ or H₂O.

According to another embodiment, a sensor head is provided for use in agas exchange analysis system, the sensor head including an active flowsplitting device having a first output port and a second output port,the active flow splitting device configured to variably split anincoming gas flow between the first and second output ports, a samplechamber having an inlet and an outlet, the inlet coupled with the firstoutput port of the active flow splitting device, a first gas analyzercoupled with the outlet of the sample chamber and configured to measurea concentration of a gas, a second gas analyzer coupled with the secondoutput port of the active flow splitting device and configured tomeasure a concentration of said gas, a flow meter coupled between thefirst output port of the active flow splitting device and the inlet ofthe sample chamber or between the second output port of the active flowsplitting device and the second gas analyzer, the flow meter configuredto measure a flow rate, and a feedback control circuit adapted tocontrol the active flow splitting device to adjust the incoming gas flowto the first and second output ports responsive to a flow rate signalfrom the flow meter.

According to another embodiment, a method of measuring gas exchange isdisclosed. The method includes flowing gas through an active flowsplitting device having a first output port and a second output port,the active flow splitting device configured to variably split anincoming gas flow between the first and second output ports, flowing gasfrom the first output port of the active flow splitting device to asample chamber having an inlet and an outlet, determining aconcentration of gas flowing from the outlet of the sample chamber,determining a concentration of gas flowing from the second output portof the active flow splitting device, measuring a flow rate between thefirst output port of the active flow splitting device and the inlet ofthe sample chamber or between the second output port of the active flowsplitting device and the second gas analyzer, and sending feedback tothe active flow splitting device based on the measured flow rate.

Reference to the remaining portions of the specification, including thedrawings and claims, will realize other features and advantages of thepresent invention. Further features and advantages of the presentinvention, as well as the structure and operation of various embodimentsof the present invention, are described in detail below with respect tothe accompanying drawings. In the drawings, like reference numbersindicate identical or functionally similar elements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a illustrates the flow path in a prior photosynthesis measurementsystem where the flow is split at the console, remote from the sensorhead.

FIG. 1 b illustrates a flow path in a photosynthesis measurement systemcording to one embodiment.

FIG. 2 illustrates a sensor head with a leaf chamber blocked off.

FIG. 3 shows the results of the experiment with the configuration inFIG. 2, using the experimental protocol outlined in the LI-CORBiosciences LI-6400 User Manual (pp 4-43 through 4-48).

FIG. 4 illustrates a different perspective view of the sensor head ofFIG. 2.

FIGS. 5 and 6 illustrate results of an experiment that shows asignificant reduction in the CO₂ concentration difference (Cs−Cr)between the sample and reference.

FIG. 7 illustrates a method of measuring a concentration differential ofa gas in a gas exchange analysis system according to one embodiment.

DETAILED DESCRIPTION

The present invention provides systems and methods for measuringphotosynthesis and transpiration rates, systems and methods forperforming survey measurements of gas concentration and also gasanalyzers having piezoresistive or other active flow splitting devicesthat variably split a received gas flow to two gas outlet ports orchannels and can be controlled precisely through feedback loops.

“Active” devices include those that can be controlled electronically bya computer, controller, or other machine. Active devices often can beadjusted in real-time automatically without intervention by a humanoperator.

Technical advantages of using a piezoelectric flow splitter include thatthey can operate with very low power consumption, which is beneficialfor battery-powered instruments. Physical size of piezoelectric flowsplitters is relatively small, and geometries of piezo actuators arefavorable for small instruments.

A closed loop feedback controller for a flow splitter has been shown tohave superior performance for gas exchange analysis systems that measuredelicate photosynthesis affects of real-world plants. The flow on oneside of a flow split into a leaf measurement chamber should be preciselyknown and controlled in order for evapotransporation to be accuratelymeasured within the chamber. Flow rate is one of the parameters of aleaf photosynthesis measurement. An flow splitter without feedback(i.e., open loop) might drift because conditions in the leaf chambermight change. The changing conditions can include temperature, partialpressures of certain gases, etc. These changes can include a pressurechange that could affect flow rate through the chamber.

In some embodiments, it is desirable to adjust the amount of flow goinginto the leaf measurement chamber to accommodate different measurementconditions. However, since the amount of flow before the flow split isusually constant, changing the split ratio has been found to be a goodway to regulate and control the flow to the sample chamber. One cancontrol the flow into the sample chamber because it should be preciselyknown and regulated. Excess flow that is left over can be used for thereference path. Having the flow splitter located very close to orotherwise proximate to the chamber paths has been found to largelyeliminate adsorption effects of the chamber, hose, and fitting walls.

FIG. 1 b illustrates a flow path in a gas exchange measurement system 10according to one embodiment. Gas exchange measurement system 10 in oneembodiment includes a console 15 and a sensor head 20 remote fromconsole 15. Console 15 typically includes, or is connected with, one ormore gas sources and gas conditioning equipment. For example, in thecontext of photosynthesis and transpiration measurements, gas sourceswould include reservoirs of CO₂ and H₂O, and conditioning equipment forconditioning each gas concentration. A flow path 17 connecting console15 with sensor head 20 typically includes flexible tubing andconnectors. Flow path 17 provides a single stream or gas flow path toflow splitting mechanism 25 in sensor head 20. Flow splitting mechanism25 receives a stream of gas from console 15 and splits the flow into twoseparate flow paths as will be described in more detail below. Onestream is provided to the chamber 30 (e.g., sample stream) and the otherstream (e.g., reference stream) is provided to a reference gas analyzer50. A second gas analyzer 40 receives and analyzes gas from chamber 30.Reference gas analyzer 50 and second gas analyzer 40 might each includean Infra-Red Gas Analyzer (IRGA), as is known in the art, or other gasanalyzer.

It is desirable that flow path lengths and the number of connectionsdownstream of the flow split device 25 be minimized to reduce parasiticsources and sinks which differentially affect concentrations in the twoflow paths. Hence, according to one embodiment, the flow path is splitin the sensor head proximal to the sample chamber. The majority ofparasitic sources and sinks, which are located upstream of the sensorhead in FIG. 1 b, affect only a single air stream (flow path 17) whenthe flow is split at the sensor head 20. Parasitic sources and sinkswhich impact the sample and reference streams independently areadvantageously minimized.

It is desirable that for a certain flow rate, through either thereference or sample path, less than a certain amount of diffusionoccurs. As one example, for embodiments of the present invention, it isdesirable that 0.1.μmole/mole (PPM) or less of CO2 concentration changeoccur at a flow rate of about 50 μmol/sec in the sample leg. Thiscorresponds to an effective diffusion rate of CO² of 5 μmoles/sec. For agiven diffusion source/sink rate, as the flow rate increases, theconcentration change due to the diffusion source/sink has a smallerconcentration effect; concomitantly, a given diffusion source/sink has agreater effect on concentration at a smaller flow rate. Hence, as above,it is desirable to minimize the flow path lengths havingdiffusion-susceptible material and components to reduce or minimizeparasitic sources and sinks of gases. One way to do this is to minimizethe flow path length itself. An additional, or alternate, way is toreduce or minimize components of the flow path that arediffusion-susceptible. In practice, however, certaindiffusion-susceptible materials and components are used for cost andefficiency reasons. Therefore, according to one embodiment, the flow issplit as close to the sample chamber and gas analyzers as possible. Incertain aspects, the flow splitting mechanism 25 is located such that aminimal amount of flow path having components or surface areas exposedor susceptible to diffusion exists between the flow splitting device 25and the sample chamber 30. The desired length of the flow path isgenerally a function of the flow rate and the diffusion susceptiblematerial or components making up the flow path; for example, for metaltubing, the flow path can be significantly longer than for plastic orother diffusion-susceptible components. For example, in certain aspects,a flow path having 12″ or less of diffusion-susceptible tubing and/orother components is desirable to couple the flow splitting mechanism 25with the sample chamber 30 to provide a gas stream flow path from thesplitting mechanism. In other aspects, less than about 6″, or 4″ or 2″or even 1″ or less of such diffusion-susceptible flow path existsbetween the flow splitting device 25 and the sample chamber 30.

Similarly, in certain aspects, the flow splitting mechanism is locatedin the sensor 30 head such that less than about 12″ of suchdiffusion-susceptible flow path exists between the flow splitting device25 and the reference gas analyzer 50. In other aspects, the flowsplitting mechanism is located such that less than about 6″, or 4″ or 2″or even 1″ or less of such flow path exists between the flow splittingdevice 25 and the reference gas analyzer 50. It is also desirable thatthat flow path length between the sample chamber 30 and sample gasanalyzer 40 be minimized. One skilled in the art will appreciate thatthe diffusion-susceptible flow path from the flow splitting mechanism 25to the reference gas analyzer 50 can be roughly the same length as thediffusion-susceptible flow path from the splitting mechanism 25 throughthe sample chamber 30 to the sample gas analyzer 40. Alternately, thetwo diffusion-S susceptible flow paths can be different lengths asdesired.

Experiments using the LI-COR Biosciences LI-6400 have verified thatdiffusion through flexible tubing, gaskets, and pneumatic connectors isa significant and quantifiable issue. In one experiment, to eliminatediffusion sources/sinks from the leaf chamber, the chamber was removedfrom the LI-6400 head (see, e.g., FIG. 1 a) and was replaced with analuminum block and a vinyl gasket as shown in FIG. 2. FIG. 4 illustratesa different perspective view of the sensor head of FIG. 2. FIG. 3 showsthe results of the experiment with the configuration in FIG. 2, usingthe experimental protocol outlined in the LI-6400 User Manual (pp 4-43through 4-48). FIG. 3 shows there are significant differences betweenthe sample (Cs) and reference (Cr) concentrations of CO₂ in the absenceof a leaf The differences shown in FIG. 3 are due exclusively toparasitic sources and sinks of CO₂. As there were no known bulk leaksduring the experiment, the differences shown in FIG. 3 are dominated bydiffusion. The magnitude of the concentration difference is largest atlow flow rates and at large CO₂ concentrations (relative to ambient Ca).

FIG. 3 demonstrates that the diffusive parasitic sources and/or sinkscan be attributed to system components other than the leaf chamber andleaf gaskets. The diffusion source and/or sink must be present in thetubing, pneumatic connectors and fittings that supply the head with thesample and reference gas streams.

A difference in sample and reference flow rates causes a difference inconcentrations even if the diffusive rates are approximately equal. FIG.3 shows that at low sample flow rates, and a given stream concentration,the concentration difference (Cs−Cr) is more pronounced than at highersample flow rates. For example, at a nominal concentration of 1913.9 PPMof CO₂, (Cs−Cr) is approximately −2.6 PPM at the lowest flow rate.Ambient CO₂ concentration is approximately 384 PPM. Under these flowconditions, the sample flow rate is much lower than the reference flowrate, and diffusion reduces the concentration of the sample much morethan the reference, resulting in the negative value of (Cs−Cr). Thereduction occurs because of diffusive parasitic sinks, whereas at anominal concentration of 35.5 PPM (below ambient Ca of 388 PPM), thevalues of (Cs−Cr) are positive, and parasitic diffusion acts as asource.

Splitting the flow at the sensor head, according to one embodiment,reduces the number of components (e.g., tubing, connectors andfittings), and the pneumatic path length, subjected to parasitic CO₂sources and/or sinks, thereby reducing the magnitude of the differencein (Cs−Cr) due to systematic issues. The advent of smaller pneumaticcomponents, including MEMS-based mass flow meters, has enabled apractical implementation of splitting the flow path in the sensor head.One useful flow meter is produced by Omron, e.g., part number D6F-02A3.

In one embodiment, the flow splitting mechanism 25 is configured tovariably split the flow of gas in the sensor head. In particular, theflow splitting mechanism is configured to variably split a gas flowreceived at the input port 24 from a remote source (console 15) to afirst output port and to the second output port as shown in FIG. 1 b.The second output port is coupled via a flow path 27 with an inlet ofthe second IRGA 50. The first output port is coupled via a flow path 26with an inlet of the sample analysis chamber 30. A flow volume,including the measurement volume in the sample chamber 30 and the flowpath, is sufficiently small such as to reduce the time required to reacha steady state of gas concentrations in the flow volume when a flowratio to the flow path is adjusted in the flow splitting mechanism. Forexample, the measurement volume might be on the order of 1 mL to 10 mLto about 1000 mL, such that the flow path including the flow volumebetween the flow splitting device and the sample IRGA 40 might besmaller than, or on the order of, about 20 mL to about 1000 mL. Incertain aspects, the flow splitting mechanism is configured to adjustthe flow ratio such that the gas flow can be controllably, andcontinuously, varied to provide a flow range of between about 0% to 100%to the first output port and the remaining 100% to 0% to the secondoutput port. For example, the flow splitting mechanism can be controlledvia a control signal to split the flow 25% to one of the first port orsecond output port and 75% to the other output port, or 50% to oneoutput port and 50% to the other output port.

Experiments were conducted with an approximate 50%-50% flow split (50%of flow to reference, 50% to sample) and a 75%-25% flow split (75% toreference, 25% to sample) at the head of a LI-6400 instrument. Theresults (FIG. 5 and FIG. 6) show a significant reduction in the CO₂concentration difference (Cs−Cr) between the sample and reference. TheCO₂ concentration inside the IRGA was roughly 1940 tmol/mol for theexperiments of FIG. 5 and FIG. 6. Comparing FIG. 3 (Cr=1913.9 mol/mol)with FIG. 6 demonstrates that splitting the flow at the head can reducediffusion effects by nearly an order-of-magnitude.

In one embodiment, flow splitting mechanism 25 includes a piezoresistiveflow splitting device is provided for use in a sensor head of a gasexchange analysis system. The device is configured to variably split agas flow received at an input port 24 to a first output port 26 and to asecond output port 27, wherein the first output port is coupled with theinlet of a sample chamber 30. Gas analyzer 40 is coupled with the outletof the sample chamber and is configured to measure a concentration of agas in the gas flow exiting the sample chamber, and gas analyzer 50 iscoupled with the second output port and configured to measure aconcentration of the gas in the gas flow exiting the flow splittingdevice. Also included is a flow meter coupled between the output port 26and the inlet of the sample chamber 30 or between the output port 27 andthe gas analyzer 50, the flow meter being adapted to measure a flow ratein the flow path, and a feedback control circuit adapted to control thepiezoresistive flow splitting device to adjust a ratio of gas flow tothe first and second output ports responsive to a flow rate signal fromthe flow meter.

In certain aspects, the piezoresistive flow splitting device includes apiezoresistive actuator having a first end secured within the device anda second end located proximal both the output port 26 and the outputport 27, and electrical contacts for providing a control potential tothe actuator to control the position of the second end relative to theoutput ports and thereby control the flow ratio to the output ports. Forexample, in certain aspects, an applied control potential controls thesecond end to adjust the position of the second end relative to theoutput ports such as to controllably adjust a flow ratio of betweenabout 0% to 100% to the output port 26 and concomitantly between about100% to 0% to the output port 27. In one embodiment, the actuatorincludes a metal strip coated on both sides with a piezo-bendermaterial. In certain aspects, the piezo-bender material includes lithiumtantalite or other piezo-resistive material known to one skilled in theart.

FIG. 7 illustrates a method 100 of measuring a concentrationdifferential of a gas in a gas exchange analysis system according to oneembodiment. The gas exchange analysis system includes a sensor headhaving a flow splitting mechanism located proximal to a sample chamberthat defines a measurement volume for analysis of a sample. The samplechamber includes an inlet and an outlet, with the inlet being connected,in close proximity, with an output port of the flow splitting device.The outlet is connected, also preferably in close proximity, with a gasanalyzer such as an IRGA. In step 110, a gas flow received from a remotesource at an input port of the flow splitting mechanism is controllablysplit to a first output port and to a second output port, with the firstoutput port being coupled with the inlet of the sample chamber. In step120, a first concentration of one or more gases exiting the samplechamber is measured using a first gas analyzer (e.g., gas analyzer 40)fluidly coupled with an output of the sample chamber. In step 130, asecond concentration of the one ore more gases exiting the second outputport is measured using a second gas analyzer (e.g., gas analyzer 50)fluidly coupled with the second output port of the flow splittingdevice. In step 140, a concentration differential of the one or moregases is determined based on the first measured concentration and thesecond measured concentration. Step 140 can be performed using aprocessor or computer system that is integrated in the sensor headand/or in the console of the gas analysis system and/or in a remotecomputer system that is communicably coupled with the gas analysissystem. In step 150, the concentration differential is output, e.g.,displayed on a monitor or other output device, printed, stored, orotherwise provided to another computer system or device. Advantageously,measurement error associated with diffusion sources and sinks of the gasare reduced due to the proximity of the flow splitting mechanism withthe sample chamber.

In another embodiment, an open-path gas exchange analysis systemincludes a first gas analyzer (e.g., IRGA) configured to measure a firstconcentration of a gas entering the gas inlet port of the sample chamberat a plurality of times and a second gas analyzer (e.g., IRGA)configured to measure a second concentration of the gas exiting the gasoutlet port of the sample chamber at the plurality of times. Theenclosed sample chamber defines a measurement volume for analysis of asample, where the gas inlet port of the sample chamber is coupled with agas source. The system also includes a processing module, communicablycoupled with the first and second gas analyzers, configured to determineat each of the plurality of times a concentration differential betweenthe first measured concentration and the second measured concentration,and to integrate the concentration differential over time. The results(data) can be output, displayed or otherwise provided to another systemor device for further manipulation. According to one embodiment, amethod of measuring a concentration of a gas in such a gas exchangeanalysis system includes measuring a first concentration of a gas at theinput port of the sample chamber at each of a plurality of times,measuring a second concentration of the gas at the output port of thesample chamber at each of said plurality of times, and thereafterdetermining at each of the plurality of times a concentrationdifferential between the first measured concentration and the secondmeasured concentration, and integrating the concentration differentialover time. In certain aspects, the gas includes CO₂ and/or H₂O.

While the invention has been described by way of example and in terms ofspecific embodiments, it is to be understood that the invention is notlimited to the disclosed embodiments. To the contrary, it is intended tocover various modifications and similar arrangements as would beapparent to those skilled in the art. Therefore, the scope of theappended claims should be accorded the broadest interpretation so as toencompass all such modifications and similar arrangements.

What is claimed is:
 1. A gas exchange analysis sensor head apparatuscomprising: an active flow splitting device having a first output portand a second output port, the active flow splitting device configured tovariably split an incoming gas flow between the first and second outputports; a sample chamber having an inlet and an outlet, the inlet coupledwith the first output port of the active flow splitting device; a firstgas analyzer coupled with the outlet of the sample chamber andconfigured to measure a concentration of a gas; a second gas analyzercoupled with the second output port of the active flow splitting deviceand configured to measure a concentration of said gas; a flow metercoupled between the first output port of the active flow splittingdevice and the inlet of the sample chamber or between the second outputport of the active flow splitting device and the second gas analyzer,the flow meter configured to measure a flow rate; and a feedback controlcircuit adapted to control the active flow splitting device to adjustthe incoming gas flow to the first and second output ports responsive toa flow rate signal from the flow meter.
 2. The apparatus of claim 1,wherein flow meter is coupled between the first output port of theactive flow splitting device and the inlet of the sample chamber, thefeedback control circuit controlling gas flow based on flow to thesample chamber.
 3. The apparatus of claim 2, wherein measurements fromthe first and second gas analyzers are configured to be compared,thereby comparing gas analyzed from the sample chamber with excess gasdiverted by the flow splitting device.
 4. The apparatus of claim 2,wherein the feedback control circuit controls a gas flow to auser-selected flow rate.
 5. The apparatus of claim 1, wherein the activeflow splitting device includes a piezoelectric material.
 6. Theapparatus of claim 5, wherein the active flow splitting devicecomprises: a piezoresistive actuator having a first end secured withinthe active flow splitting device and a second end located proximal toboth the first and second output ports of the active flow splittingdevice; and electrical contacts operatively coupled with thepiezoresistive actuator, configured to apply a voltage to control aposition of the second end relative to the first and second output portsand thereby control the flow ratio to the first and second output ports.7. The apparatus of claim 6, wherein the actuator includes a metal stripcoated on at least one side with a piezo-bender material.
 8. Theapparatus of claim 7, wherein the piezo-bender material compriseslithium tantalite.
 9. The sensor head of claim 6, wherein an appliedcontrol potential controls the second end of the piezoresistive actuatorto adjust the position of the second end relative to the first andsecond output ports such as to controllably adjust a flow ratio ofbetween about 0% to 100% to the first output port and concomitantlybetween about 100% to 0% to the second output port.
 10. The apparatus ofclaim 1, wherein the active flow splitting device is configured toadjust a flow ratio of the flow meter such that an incoming gas flow issplit between about 0% to 100% to the first output port andconcomitantly between about 100% to 0% to the second output port. 11.The apparatus of claim 1, wherein the active flow splitting device islocated less than about 12 inches from the sample analysis chamber
 12. Amethod of measuring gas exchange, the method comprising: flowing gasthrough an active flow splitting device having a first output port and asecond output port, the active flow splitting device configured tovariably split an incoming gas flow between the first and second outputports; flowing gas from the first output port of the active flowsplitting device to a sample chamber having an inlet and an outlet;determining a concentration of gas flowing from the outlet of the samplechamber; determining a concentration of gas flowing from the secondoutput port of the active flow splitting device; measuring a flow ratebetween the first output port of the active flow splitting device andthe inlet of the sample chamber or between the second output port of theactive flow splitting device and the second gas analyzer; and sendingfeedback to the active flow splitting device based on the measured flowrate.
 13. The method of claim 12, wherein the active flow splittingdevice includes a piezoelectric element.
 14. The method of claim 12,wherein the feedback controls the active flow splitting device toestablish a user-selected ratio.
 15. A method of measuring aconcentration of a gas in a gas exchange analysis system having a samplechamber defining a measurement volume for analysis of a sample, thesample chamber having an inlet port coupled with a gas source and anoutlet port, the method comprising: measuring a first concentration of agas at the input port at each of a plurality of times; measuring asecond concentration of said gas at the output port at each of saidplurality of times; and thereafter determining at each of said pluralityof times a concentration differential between the first measuredconcentration and the second measured concentration; and integrating theconcentration differential over time.
 16. The method of claim 15,further comprising: using the integrated concentration differential tocalculate an evapotranspiration rate.
 17. The method of claim 15,wherein said gas includes CO₂ or H₂O.
 18. An open-path gas exchangeanalysis system, the system comprising: an enclosed sample chamberhaving a gas inlet port and a gas outlet port; a first gas analyzerconfigured to measure a first concentration of a gas entering the gasinlet port a plurality of times; a second gas analyzer configured tomeasure a second concentration of said gas exiting the gas outlet portat the plurality of times; and a processing module configured todetermine at each of said plurality of times a concentrationdifferential between the first measured concentration and the secondmeasured concentration, and to integrate the concentration differentialover time.
 19. The method of claim 18, further comprising: using theintegrated concentration differential to calculate an evapotranspirationrate.
 20. The system of claim 18, wherein said gas includes CO₂ or H₂O.